Field of the Invention
The present invention relates to an image forming apparatus having a function to supply lubricant onto an image bearing body, and also relates to an image forming method on the image forming apparatus.
Description of the Related Art
Electrophotographic type image forming apparatuses such as a multi-functional peripheral, copier, and a printer have been widely used. The electrophotographic type image forming apparatus as above typically includes an image bearing body, a transfer apparatus, and a cleaning member. The image bearing body is a component on which a toner image is formed while the image bearing body is being rotationally driven. The transfer apparatus transfers the formed toner image to a transfer body or a medium. The cleaning member collects, after the transfer of the toner image, the residual toner adhered to a surface of the image bearing body and then cleans the surface of the image bearing body.
As the image bearing body, a photoreceptor is employed. For the photoreceptor, execution of a cycle including a charging step, an exposure step, and a developing step, is repeated. In the charging step, a surface of the photoreceptor is charged uniformly. In the exposure step, a surface of the photoreceptor is exposed according to a specified image pattern so as to form an electrostatic latent image. In the developing step, toner is supplied to the surface of the photoreceptor so as to develop the electrostatic latent image.
In addition, it is typical that a lubricant supply mechanism is provided. The lubricant supply mechanism supplies lubricant onto the image bearing body for decreasing a frictional force generated between the cleaning member and the image bearing body. As a typical lubricant, a metal soap such as a metal stearate is employed. Various types of lubricant supply mechanisms are known. One type has an application mechanism including a brush, provided upstream or downstream of the cleaning member. Another type adds lubricant in the toner and supplies the lubricant at the developing unit. And yet another type combines these two types. In a configuration with the lubricant supply mechanism, lubricant is applied to a surface of the image bearing body, leading to a lowered friction coefficient for the toner on the surface of the image bearing body. The lowered friction coefficient suppresses a defective transfer when a toner image formed on the surface of the image bearing body is transferred to the transfer material, or the like. Accordingly, it is possible to improve image quality of the toner image. Moreover, this configuration also lowers the friction coefficient between the image bearing body and a member (cleaning blade, for example) that is press-contacted against the image bearing body. This is effectively suppresses wear (scraping) on the surface of the image bearing body, making it possible to extend a service life of the image bearing body.
The lubricant supplied by the lubricant supply mechanism and a lubricant layer (lubricant coating) formed by the lubricant are degraded by discharge products generated in the charging step, or the like. At the same time, the lubricant itself is also degraded by own deterioration. This kind of degradation sometimes causes an image flow due to reduced resistance of the lubricant layer, or abnormal wear of the cleaning member due to loss of lubricity (effect of reducing the frictional force) of the lubricant.
Moreover, the amount of lubricant existing on the image bearing body varies depending on a B/W ratio (black/white proportion) of an image pattern as a printing target, or depending on an installation environment of the image forming apparatus. The varying amount of lubricant applied sometimes causes an increase in the frictional force. This sometimes increases likelihood of wear of the cleaning member or lowers cleaning performance.
Therefore, a system employing a lubricant supply mechanism is required to provide a configuration to appropriately refresh lubricant, namely, a configuration to scrape (remove) old lubricant on the image bearing body and re-apply lubricant. Along with this, the system is required to provide a configuration to properly maintain the amount of applied lubricant on the image bearing body. Some known techniques employ a series of operation (hereinafter, also referred to as a “refresh mode”) of removing degraded lubricant and supplying lubricant.
Execution time of the refresh mode is typically managed based on a predetermined time or the number of pulses, that can be predicted experimentally. For example, JP 2002-006689 A discloses an image forming apparatus that supplies lubricant to an image bearing body that forms a toner image so as to extend it service life and improve image quality. In a specific configuration, the image forming apparatus disclosed in JP 2002-006689 A, in order to remove discharge products on a photoreceptor, temporarily collects lubricant on the photoreceptor, so as to increase friction coefficient, and then, supplies lubricant. JP 2002-006689 A defines collecting lubricant from on the photoreceptor as a refresh mode, and the refresh mode is executed for a predetermined time.
Another known system has a configuration to supply lubricant by applying the lubricant. For example, JP 2005-181742 A disclose an image forming apparatus having a control means that, when it detects a photoreceptor unit as a new unit, performs application operation using a lubricant application means, measures a photoreceptor torque, and determines application operation time. In other words, JP 2005-181742 A discloses a configuration to apply lubricant while detecting a dynamic frictional force of the photoreceptor. Similarly, JP H08-305236 A discloses a configuration to detect changes in a pressure roller (charging roller and lubricant application roller) press-contacted against an image bearing body, such as a change in a rotation speed, operation torque, and an operation current value, using a detection means. With a detection signal, the configuration then controls, by using a control means, the amount of applied lubricant with a lubricant application means.
Unfortunately, however, if the refresh mode is executed under a predetermined condition after consecutively printing a large amount of materials based on an image pattern having a B/W ratio largely different from an ordinary B/W ratio of several %, or after an installation environment of the image forming apparatus has been changed, execution of the refresh mode might fails. In other words, executing the refresh mode for a predetermined limited and fixed period of time would turn out to be insufficient refreshed state due to insufficient removal of the lubricant, or overly refreshed state due to the excessive amount of collected lubricant. For example, in an environment with high temperature and high humidity, it is possible that a large amount of lubricant exists on an image bearing body, leading to insufficient cleaning.
Regarding these viewpoints, how the amount of applied lubricant can be optimized, or the like, is not taken into account in JP 2002-006689 A. Meanwhile, in JP 2005-181742 A and JP H08-305236 A, a situation in which a large amount of lubricant exists has not been taken into account.